Peel and thinly slice the yellow onions.
Melt butter in a large pot over medium heat. Add the sliced onions and cook, stirring occasionally, until deeply caramelized.
Crush or mince the garlic and stir it into the caramelized onions.
Sprinkle the flour over the onion mixture and stir well to combine, cooking for a minute.
Pour in the white wine to deglaze the pot, scraping up any browned bits from the bottom.
Add the beef broth, Worcestershire sauce, black pepper, salt, fresh thyme, and bay leaf. Cover and let it simmer.
Ladle the hot soup into oven-safe bowls.
Top each bowl with a slice of baguette and a generous amount of shredded Gruyere cheese.
Bake in the oven until the cheese is melted, bubbly, and slightly browned on top.
Garnish with a little fresh thyme and serve immediately.
Cook it hands-free.
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